Ecology of Animals 667 



ing corals, bryozoans, crinoids, sea pens, and tunicates. In addition there 

 are many sea cucumbers, molluscs, pycnogonids, and long-legged crabs. 

 Above these last two zones is the oceanic province of the nektonic 

 and planktonic forms of the open sea. 



POPULATIONS 



Ecology is essentially a study of populations, their growth, inter- 

 relationships, and limits. In short, it is a study of their dynamics. 



Every community has within it a complex of interrelated popula- 

 tions. In any natural community the animals and plants are in balance 

 with one another. Any increase in numbers of some one species is 

 soon checked. This check may be a rise in numbers of predators, a 

 decrease in food, or even some disease. By such means, the numbers are 

 kept within normal bounds. 



At times this vital balance is upset by man's interference. In such 

 cases, certain species may increase in numbers due to the removal of 

 the natural limiting factors. Eventually a new balance may be reached, 

 but in the meantime the whole community is upset. 



Growth of Populations. — It is difficult to observe population 

 growth under natural conditions, so most information has been obtained 

 from observations made under controlled conditions in laboratories. 

 Studies have been made on protozoans, Drosophila, bacteria, and the 

 flour beetle, Tribolium, among many others. 



The initial growth under such controlled conditions is slow, but 

 as the numbers of breeding individuals increase, the number of indi- 

 viduals in the population increases rapidly to a maximum and then 

 levels off. If this growth were to be plotted on a graph, it would be 

 seen that an S-shaped curve resulted. This type of curve, known as 

 a sigmoid curve, is typical of population growths. The upper limit 

 varies with different limiting factors. Most often these factors are 

 food or space, but such things as predators, disease, or change of substrate 

 can also affect the growth. 



Without benefit of laboratory experimentation, Malthus long ago 

 postulated that human populations increased geometrically while the 

 food supplies increased arithmetically. When the population became 

 too great for the food supply, he stated, starvation w^ould result in a 

 population decrease. Essentially this does fit the picture of population 

 growth; however, since Malthus's time, technological improvements 

 have constantly raised the upper limit of food production. Human 



